Ford engine coolant temperature sensors, or ECTs, are devices designed to measure the temperature of the liquid coolant passing through the Ford engine block. In this way, the vehicle can get some idea of the engine temperature and adjust both coolant and fuel flow accordingly.
Most modern Fords have the ECT mounted in the neck of the output valve at the back of the engine, where coolant leaves the engine to head back to the radiator. A strip of semi-conductive ceramic or polymer material is suspended in the center of the outlet valve's neck ring. On each end of the strip is a wire lead which runs back to the onboard computer in modern vehicles or the powertrain control module in later model Fords. The onboard computer or PCM is typically located in the engine compartment on the passenger side on most Fords.
When the Ford is started, coolant passes over the strip of semi-conductive material. A steady current is passed from the computer or PCM through the strip and back. The Ford ECT is technically considered a thermistor in that its resistance to conducting electricity changes as it heats up. When the engine is cold, most of the electricity passing through the ECT is lost due to resistance. The PCM or onboard computer interprets this lack of return electricity as a cold engine and increases the gas in the fuel injector's fuel-air ratio for a richer mix to ensure the engine keeps running. As the car heats up, so does the coolant passing through it and the ECT. This allows a greater amount of electricity to pass back to the PCM or onboard computer. The response is then to thin the fuel mixture to keep the engine from overheating.
